Motorcycles generally consist of a main frame and a rear frame. This rear frame is commonly called a swing arm, although it is more than a single arm. The swing arm supports the rear tire of the motorcycle. A suspension system is typically used to attach the swing arm to the main frame. The suspension system permits independent vertical movement of the swing arm relative to the main frame. This independent movement allows the wheel to respond to road fluctuations and variances in terrain. The suspension system also prevents forces caused by bumps and jolts in the road from being transmitted to the main frame and rider. This improves the handling quality and riding comfort of the motorcycle.
A motorcycle suspension system commonly consists of an arm or fork extending from each wheel attachment point or wheel mount located on each side of the rear wheel. The axle of the rear wheel is connected to the wheel mounts. The rear wheel rotates about the axle on a wheel axis. The other ends of these arms are attached to the main frame by a suspension system in a manner that allows vertical movement of the arms relative to the main frame of the motorcycle. A typical suspension system includes many components such as arms, links, shock absorbers, springs, or torsion bars. These components are usually positioned between the arms and the main frame to permit the movement of the wheel relative to the main frame.
For example, U.S. Pat. No. 4,913,255 discloses a shock absorption system for motorcycles that includes a torsion spring. One end of the torsion spring is fastened to the lower frame of the motorcycle while the other end is fastened to the swing arm. A rotary dampener is also positioned between the swing arm and the frame. The torsion spring and the rotary dampener are physically positioned apart from each other, but they work in concert to form a suspension system. This device has the disadvantages of requiring a plurality of complex, interconnected, and exposed parts to form suspension system.
U.S. Pat. No. 4,463,824 discloses a suspension system for a motorcycle which employs a pair of arms located on each side of the rear wheel. These arms are connected at one end by a link to the wheel mount. The other end of the arms are attached to the main frame by another link in a manner to form a parallelogram. A torsion bar is connected at one end to the linked arms and the other end to the motorcycle frame. This device has the disadvantage of requiring a plurality of components and pivot points to create a suspension system.
There remains a need for a suspension system which is simple, easy to manufacture and install, and does not require a plurality of parts.
In addition, it is often highly desirable to have a suspension system that has no visible suspension components. Harley-Davidson is a well known motorcycle manufacturer that constructs a very popular type of suspension system under the Softail name. The Softail suspension has no suspension components exposed to view and is particularly designed to have the appearance of Harley-Davidson's "hard tail" frame. The "hard tail" frame consists of a pair of angularly extending arms joined to a wheel mount, with a wheel mount located on each side of the rear wheel. The arms are spaced from each other to allow clearance of the rear wheel. The other ends of the arms are connected directly to the frame strut of the motorcycle main frame. There is virtually no suspension system to absorb deflection of the rear wheel. Thus, all movement caused by the rear wheel hitting bumps in the road is transmitted to the main frame and the rider. This severely impairs the riding and handling qualities of the motorcycle.
The Harley-Davidson Softail suspension system attempts to mimic the appearance of the "hard tail" frame while improving the riding quality and comfort of the motorcycle by incorporating a suspension system. Instead of connecting the arms extending from the wheel mounts directly to the frame, each arm is attached to a connector. The connectors have the same general size and shape as the frame struts of the main frame. Each connector is positioned adjacent and inside a corresponding frame strut so they are hidden from view. The connectors are pivotally attached to the frame struts along a pivot axis. The connectors pivot about a simple shaft or swing axle tube which extends along the pivot axis. The pivot axis is generally located near the center of the connectors. The connectors are attached to each other by upper and lower transverse cross members at the respective ends of the connectors. The Softail suspension does not alter the aesthetics of the "hard tail" frame because the connectors are hidden from view by the frame struts.
The Softail suspension system requires the use of two shock absorbers mounted parallel to the bottom of the motorcycle main frame. The shock absorbers are hidden from view when mounted in this location. One end of each shock absorber is attached to the main frame and the other end is connected to a projection, extending forwardly from the lower transverse cross member of the swing arm. The projection is adapted to receive a bolt or pin which connects the shock absorber to the projection. The main frame of the motorcycle hides the projection from view. Therefore, this arrangement and location of the shock absorbers and projection retains the image of no suspension system, which preserves the highly desirable appearance of a Harley-Davidson "hard tail" frame.
The Softail suspension system affords significant advantages over the "hard tail" frame by providing for some suspension cushioning that improves the handling and riding comfort of the motorcycle. However, significant disadvantages remain because the Softail suspension system is insufficient. For example, a harsh and uncomfortable ride remains because of the very short travel of the shock absorbers. The shock absorbers have a very short travel because of the design and geometry of the Harley-Davidson motorcycle frame. The rear wheel can only move a maximum of two inches in either vertical direction before the swing arm contacts the main frame. Thus, the maximum vertical movement of the rear wheel relative to the main frame is limited.
The shock absorbers, however, have an even smaller maximum travel of 1.5 inches, 0.75 inches in either direction from a center position. As described above, the shock absorbers are attached to the lower portion of the main frame and to a projection extending from the lower transverse cross member of the swing arm. The pivot axis is about 16 inches forward of the wheel axis. The distance between the pivot axis and the shock attachment point of the projection is only about 5 inches, and that attachment point is about 1/2 inch forward from the pivot axis. The result of this is that while the rear wheel may travel two inches in one direction from a centered position, the shock absorbers can only travel a distance of about 75 inches.
The limited distance of travel of the shock absorbers results in a hard, uncomfortable ride because any shock absorption must occur within this narrow range of movement. The short distance of travel mandates continual and rapid changes of the shock absorber shaft according to the movement of the wheel. These directional changes, however, are delayed because of the inherent reluctance of the shock absorber to rapidly change directions. Shock absorbers have intrinsic parasitic and inertial drag caused by the movement of the oil or gas within the shock absorber, friction and "sticktion" caused by the seals and o-rings, and relatively large momentum forces. Thus, the Harley-Davidson Softail suspension has significant disadvantages because its suspension system is inadequate.
Previous attempts to improve the Softail suspension system have occurred. For example, the travel distance of the shock absorbers has been increased. However, this solution offers only a modest improvement because the range of movement of the swing arm is limited by the motorcycle frame. The maximum deflection of the rear wheel occurs when the swing arm contacts the frame. Because the shock absorber is also attached to the swing arm, its amount of travel is similarly limited. Any further increases in the shock absorber travel require modifications to the motorcycle main frame, which is very undesirable. Additionally, attempts have been made to improve the responsiveness of the shock absorbers, however, these attempts are hindered by drag and friction forces discussed above.
Therefore, there remains a need for a motorcycle suspension system that is largely not visible and does not have the above-described significant disadvantages.